Fix Google Sign-In deserialization crash on desktop targets under ProGuard (#16)

[sunildhiman90]

Resolves #16

Cause of the Crash

When packaging a Compose Multiplatform desktop application for distribution (e.g., as .msi, .dmg, or .deb), ProGuard runs by default on the JVM target to shrink the bundle size and optimize code.

In GoogleAuthManagerJvm, the library was using Gson to deserialize the Google user info JSON response into a private nested data class: GoogleAuthManagerJvm.GoogleUser. Since Gson relies on runtime reflection, it fails in a shrunken/obfuscated environment because ProGuard strips/renames the fields and constructors of private classes, leading to the reported RuntimeException.

Solution

• Switched from Gson to Kotlinx Serialization for parsing the user info response.
• Deleted the redundant, reflection-based private nested GoogleUser class and reused the shared @Serializable GoogleUser data class from commonMain.
• Since kotlinx.serialization uses compile-time generated serializers rather than runtime JVM reflection, it is fully compatible with ProGuard/R8 out of the box.
• Bumped the library version to 0.3.8.